The performance of a solar thermal power system is dependent on the magnitude of the solar flux delivered by a heliostat field. Many factors affect the delivered flux, including, for example, the soiling of heliostat mirrors by dust or other contaminants.
Measuring the reflectivity of a heliostat field is a difficult task using prior art. Standard practice is to position a handheld reflectometer on the surface of each heliostat to be tested. This process is time-consuming and expensive, but more importantly it is subject to systematic biases. The users of the device may systematically choose parts of the mirror to measure that are clearer or dirtier than average, depending on their incentives. Furthermore, such devices typically only sample a subset of the solar spectrum and do not necessarily measure the amount of light reflected in a solid angle that is most relevant to the solar thermal application.